The present invention generally relates to differential amplifier circuits and more particularly to feedback systems for controlling input and output stage amplifiers within the differential amplifier circuits.
Differential amplifiers are highly useful because of being able to handle a differential-mode signal in the presence of a common-mode signal without adverse effects to the differential-mode signal. For the case of a balanced amplifier the differential and common-mode signals are orthogonal. However, in most present day applications only the differential-mode is utilized as the common-mode signal represents an undesired d.c. or a.c. voltage that is considered to be noise or interference. It is therefore desirable to suppress the common-mode signal in the amplifier to improve the signal-to-noise ratio of the desired signal.
Prior art devices such as those disclosed in two of the inventor's previous U.S. Pat. Nos. 3,262,066 and 3,638,132, teach the gain of the common-mode signal can be reduced by using active common-mode feedback techniques in combination with either active or non-active differential-mode feedback circuits.
A drawback to such systems is that if near perfect balance of the amplifier or other components cannot be maintained, undesirable differential-mode cross-coupled terms are generated in the amplifier by the common-mode signal. These cross-coupled terms then cannot be suppressed without suppressing the entire differential-mode signal.